1. Field of the Invention
The invention relates to pressure-sensitive adhesive tapes and further to their uses on food packages.
2. Background Art
The use of microwave energy revolutionized food preparation and has now become common place. However, the advantages associated with microwave usage are tempered with compromises to food appearances, texture and sometimes ease of preparation.
To compensate for the microwave oven's inability to crisp and brown the surface of foods and to prevent foods, highly absorptive of microwave energy from being overheated, resulting in toughening and dehydration of the food, two specific "microwave active" packaging components have been developed: microwave susceptors and microwave shields.
Microwave susceptors are devices which heat in response to microwave energy, converting microwave energy into thermal energy to produce browning and/or crisping of food surfaces placed in contact with microwave susceptors. Susceptors may be constructed by a variety of methods such as vacuum metallization of polymer substrates, or conductive particles dispersed in a suitable binder. Microwave susceptors convert a portion of the incident microwave energy into sensible heat.
Microwave shields are devices that do not heat appreciably in response to microwave energy, but reflect virtually all incident microwaves. Metallic foils are generally employed as microwave shields.
Microwave active devices that are microwave absorbing or shielding are disclosed in U.S. Pat. No. 4,190,757. The food packaging described in the '757 patent includes a heating body that is a laminate of a supporting sheet and a layer of an "absorptive lossy substance." The packaging also includes a shield, such as metallic foil to reflect microwaves. However, the metallic foil can be perforated to allow a proportion of the microwave energy to reach the food.
The food packaging described in U.S. Pat. No. 4,230,924 includes a flexible sheet of dielectric material, such as polyester substrate having an ultrathin metallic coating that is subdivided into islands with nonmetallic gaps or strips therebetween. When the food package is placed in a microwave oven, some of the microwave energy passes through the sheet. A lesser amount of the microwave energy is converted into thermal energy by the metallic coating, which functions as a microwave absorber, thus permitting browning or crisping of adjacent food.
The subject matter of U.S. Pat. No. 4,267,420 is similar to that of the '924 patent, supra, but includes an additional sheet or substrate of plastic laminated to the metallic coating.
The microwave susceptor packaging described in the aforementioned patents allows the surface of packaged food to be heated by thermal energy. The combination of microwave and thermal heating permits browning and crisping times to be correlated with heating times.
An alternative use of a microwave susceptor is to vent a vapor-tight package. U.S. Pat. No. 4,640,838 describes a microwave susceptor incorporated into a tape that has a low-adhesion carrier web bearing a layer of graphite or carbon black particles and a layer of pressure-sensitive adhesive (PSA) by which a piece of the tape is adhered to a package. After the carrier web is stripped off, the package is placed in a microwave oven and heat generated in the graphite or carbon black layer automatically vents the package. Package venting is achieved by softening and weakening the package material underneath the tape. Alternatively, when the tape covers an opening in the package material below the susceptor, venting is achieved by softening and weakening the tape itself.
Generally, microwave susceptor packaging consists of aluminum vapor-coated poly(ethylene terephthalate) ("PET") substrate, adhesive laminated to paper or paperboard such as is in U.S. Pat. Nos. 4,641,005 and 4,735,513. The packaging is formed into popcorn bags, pizza trays and the like.
Microwave susceptors fashioned into a pressure sensitive tape or into an adhesive label, advantageously permit the user the flexibility of attaching the tape or label at any position on a finished product. A previously commercialized microwave susceptor label that employed an acrylic-based PSA, produced a noxious odor when the label was heated. The odor arising from heating the labels apparently caused the labels to be withdrawn from the market shortly after their introduction.